In copending applications Ser. Nos. 740,451 now abandoned and 740,452, now U.S. Pat. No. 4,093,569, a process is described for the preparation of low viscosity dispersions of polyisocyanate polyaddition products in polyhydroxyl compounds, in which the polyaddition products are produced in situ in the polyhyroxyl compound in the presence of a considerable quantity of water (preferably from 7 to 35% by weight, based on the total quantity of reaction mixture).